Update overview notebook

[CDJellen]

Summary of core changes:

• Structural changes to the overview notebook
• Improved the content describing the API's get_modes method and it's returns
• Streamlining of the sections covering the available_realtime and available_historical methods
• Corrections to the get_data section in light of the use of a multi-index in response.

These changes are thanks to #57 which highlighted the shortcomings of the current overview notebook.

Future changes:

• Improvements to the API which help answer the question "what data modes are available for a given station at a given time" without having to parse through NDBC URLs.
• Updates to flags in the methods throughout the API.
• Inclusion of station metadata in the xarray.Dataset object returned by the get_data method.